The temperature of gaseous precursors prior to their reaction in a chemical vapor deposition (CVD) reactor is an important parameter for achieving efficient deposition at the desired location, e.g., for efficiently depositing the intended semiconductor material on the intended substrate.
Consider, for example, growth of GaN by halide (hydride) vapor phase epitaxy (HVPE) processes. Certain HVPE processes react gaseous GaCl3 directly with gaseous NH3 in a CVD reactor chamber to yield GaN, which deposits on a substrate, and NH4Cl, which exhausts from the reactor. However, this direct reaction proceeds efficiently only if the precursor gases are at temperatures of about 900-930° C. and above, because at lower temperatures (it is believed) the precursor gases have not adequately decomposed into directly reactive species and thus cannot rapidly react. Also, at lower temperatures, GaCl3 and NH3 can form unreactive and undesired, adducts, e.g., GaCl3:NH3. Accordingly, significant fractions of inadequately thermalized precursor gases can be simply wasted; they pass through the reactor and out the exhaust either unreacted or as unreactive adducts. Similar effects arise in the CVD growth of other III-N compound semiconductors by HVPE and other processes, and also more generally, in the growth of III-V compound semiconductors.
Further, waste of inadequately thermalized precursors can be greater in high volume manufacturing (HVM) of III-V compounds, and the wasted fraction can increase as higher growth rates are sought. This is possibly because: higher growth rates require higher precursor flow rates; higher flow rates decrease times available for reaction in the vicinity of the intended substrate; and decreased reaction times reduce the efficiency of reaction-rate-limited growth processes.
However, problems have hindered achieving adequate thermalization of precursor gases, especially in the case of HVM of III-V compounds where corrosive precursor gases must be thermalized to high temperatures. First, it has not been possible to adequately thermalize precursor gases prior to entering the reactor chamber. Typical precursor gases at high temperatures can rapidly corrode materials commonly used in known gaseous delivery systems, and such corrosion can further lead to particle formation/deposition in delivery lines, reduced reactor cleanliness, eventual line blockage, and so forth. But use of corrosion-resistant materials, e.g., quartz, graphite, silicon carbide, etc, in gaseous delivery systems would be prohibitively expensive.
Further, it has also not been possible to achieve adequate precursor-gas thermalization within known CVD reactors after their entry into the reactor chamber. Especially in the case of HVM of GaN (and other III-V compounds), precursor gas flows can be unusually high, e.g., in excess of 50 slm (standard liters min.). At such high flow rates, precursor gases move too quickly through the high temperature zone in the reactor to become adequately thermalized prior to traversing the growth substrate.
Uses of planar radiation-absorbing materials in the interior of CVD reactor chambers have been described in, e.g., U.S. Pat. No. 6,325,858. This patent discloses use of a silicon carbide (SiC) plates placed downstream of the susceptor as “getter” plates on which unwanted deposition preferentially occurs. This patent also discloses use of SiC plates in contact with the quartz chamber walls in order to heat the walls and thereby also limit unwanted deposition. Dauelsberg et al. (Journal of Crystal Growth 298 418 (2007)) describe a perforated cover plate for a standard showerhead gas injector of the type used in MOCVD (metal organic CVD) processes for growing GaN which is stated to increase temperatures.
Thus, although proper thermalization of the precursor gases used in the HVM of III-V compounds, and especially of GaN, is important, the prior art provides no adequate teachings (of which the inventors are aware) concerning how such thermalization can be accomplished.